Method of retaining a winding element in a stator slot

ABSTRACT

Stator bars and stator coils (winding elements) for a large dynamoelectric machine which are to be permanently placed in the slots of the magnetic core of the machine are wrapped so as to be surrounded on three sides with an elongated sheet of an elastomeric substance. A stretching device is attached to the two overlapping edges of the elastomeric sheet in such a manner as to be able to stretch the sheet around the winding element and thus reduce the thickness of the elastomer. The winding element surrounded by the stretched layer of the elastomeric substance is placed in a selected stator slot and the stretching device is detached from the elastomeric material. The elastomeric material attempts to regain its former thickness but is confined to the remaining space between the winding element and the sides of the slot. The offal is trimmed from the elastomeric substance and the winding element remains lodged in the slot.

This application is a divisional application of U.S. application Ser.No. 08/627,103 filed in the United States Patent Office on Apr. 3, 1996,now U.S. Pat. No. 5,708,315.

In large dynamoelectric machines, the magnetic structure is usually madeup of punchings which are stacked so as to produce a magnetic statorcore body having a central bore. The magnetic core body also defines aseries of evenly spaced radially extending core slots opening into thebore. A series of insulated winding elements i.e. stator bars or statorcoils, having a generally rectangular cross section, are disposed in theprovided core slots, and are generally electrically connected at theirends in a predetermined arrangement to form the winding which producesthe magnetic field in the magnetic core structure.

There are usually two winding elements disposed in a single core slot,and there may be only one winding element or there may be more than twowinding elements in a single core slot in special circumstances. Thewinding elements are stacked one above the other in the slots providedand each set of winding elements so disposed are generally held in placein the slots by means of longitudinal wedge members which are driveninto longitudinal dovetail grooves provided near the top of each slot,in each side of the slots, just adjacent the stator bore. This method ofwedging the previously placed winding elements in the stator has beenstandard practice for many years and has for the most part producedsatisfactory results.

Winding elements may be held in the magnetic slots by the presence ofinsulating material applied to the magnetic structure during a standardvacuum impregnation process. At times, insulating shim members arepressed into the space between the winding element and the slot wall;sometimes wavy springs are also inserted into the sidewall space betweenthe winding element and the slot wall to secure the winding element inthe lot. Nevertheless, a slot wedge is universally used to assure thefinal position of the winding element in the slot, regardless of thepresence of other materials which may have been introduced in the spacebetween the winding element and the wall of the magnetic slot in themanufacture of the dynamoelectric machine.

It will be understood by those skilled in the art that the constructionof a magnetic core involves the placing of pre-stamped punchings in apredetermined well known manner to form a monolithic magnetic structure.

The stator lamination punchings must be carefully stacked during theassembly of the magnetic core, and despite efforts to produce aperfectly stacked magnetic body, in most instances the assemblyprocedures employed during the stacking operation lead to the productionof a completed assembly where the punchings are slightly misaligned sothat the winding slot that results in the finished magnetic core hasless width than the width of the individual slot in a single punching.The variance though very slight nevertheless exists. In addition to theslight variance in the slots, it will be found that the winding elementswhich are ultimately placed in the slots of the magnetic core will befound to have slight variations in the external dimensions due toimperfect manufacturing techniques, which may be caused by uneventhickness of the insulation applied to the winding elements, thus,machine manufacturer must allow for tolerances in both the slot widthand the imperfect winding element width, thus a perfect fit between thewinding elements and the slots in which they are disposed is seldomachieved. Those skilled in the art will recognize that an interferencefit must be designed into the system in order that the winding elementsmay be inserted into the stator slots and retained in the slots withoutsuffering damage to the groundwall insulation during the insertionprocess.

If for instance, the rotor of a dynamoelectric machine is producing amagnetic field in the stator of the dynamoelectric machine,electromagnetic forces are set up in the winding elements of the statorwhich tend to cause the winding elements to vibrate in the slots in aradial and axial direction. This vibration produces wear on theinsulation surfaces on the sides of the winding elements, as well on thesurfaces exposed to the wedging system. This may also lead to theproduction of added noise in the operation of the machine. Prior artefforts to improve the performance of dynamoelectric machines byapplying some kind of radial restraint to the bars, have not beenentirely satisfactory as the discussion of prior art referencesfollowing will show.

The objects of this invention are to provide an improved arrangement forsecuring stator winding elements in the slots of the magnetic structureof the machine in which they are disposed, so as to decrease theincidence of wear on the insulation surfaces of the stator bars causedby the vibration produced in the stator bars by the alternating magneticfield. The mounting arrangement will effectively dampen the vibration ofthe stator bars in the stator slots of the laminated stator core in boththe radial and the axial direction.

At the same time, the noise generated by the vibration of the statorbars in their respective stator slots is substantially reduced becauseof the reduced vibration of the stator bars.

Additionally, this invention seeks to improve the heat flow from thestator bars to the surrounding magnetic material forming the statorslots.

It is a primary object of this invention to produce an electricallyconductive ground plane in the space between the winding elements of adynamoelectric machine and the slots in the machine in which the elementis located so as to eliminate slot discharges in the spaces which mayoccur between the winding elements and the magnetic slots of prior artmachines. This invention is especially important in the elimination ofelectrical discharges in the magnetic slots in high voltage machines.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 3,155,770 to Coggeshall et al provides a coil dampeningarrangement for AC dynamoelectric machines wherein "ripple springs" areprovided to produce side wise or transverse biasing forces on the sidesof the magnetic core slot and the sides of each winding elementsdisposed in the slots. Motion of the winding elements is said to bereduced by this method of applying pressure to specific areas of thewinding elements by the wavy spring action.

This reference requires that the side ripple springs be non-magnetic andare usually a glass fiber material or stainless steel. Installation ofthe ripple springs requires time and ultimate placement is important,Coggeshall does not rely on total intimate contact between the magneticcore slot and the winding element to provide cooling of the stator bars;a cooling gas is ducted through the stator and arranged to circulatethrough the crevices where the ripple springs are placed.

U.S. Pat. No. 3,393,335 to Pletenik et al seeks to prevent the vibrationof winding elements in the slots of dynamoelectric machines. Pletenikuses an elastomeric substance inserted between the slot wedges and thewinding element to apply pressure on the winding element so as toprevent motion of the winding element in the machine slot.

U.S. Pat. No. 3,943,392 to Keuper et al provides a U-shaped slot linerof an insulating material which have spongy ribs extending along thelength of the liner. These ribs absorb a thermosetting resin at a timewhen the winding elements have been inserted in the machine slots toeffectively "lock" the winding elements and liners in the machine slot.

U.S. Pat. No. 4,369,389 to Lambrecht utilizes wavy springs under theslot wedge and adjacent winding elements to effectively apply constantpressure on the winding element to keep it in place in the slot of themachine.

U.S. Pat. No. 5,365,135 to Konrad et al uses a conformable hose filledwith a resinous liquid under pressure which is located between the slotwedges and the winding element which when pressurized and cured,provides a constant force to hold the winding elements in place.

Paper CH1717 15th EIC Insulation Conference, Chicago, Ill. Oct. 19-22,1981 by Hyndman et al describes the problems faced by electricaldesigners of AC dynamoelectric machines with the advent of stator barsinsulated with a thermoset resin based micaceous ground insulation, inwhich stator bar movement, looseness and other vibrations become areality. Another electrical phenomenon (i.e. spark erosion) is alsodiscussed.

The solution provided by the above paper is the application of a layerof electrically conductive and insulating room temperature vulcanizingmaterial to the sides of winding elements being inserted into themagnetic slots of the dynamoelectic machines. Generally the material isa silicone rubber compound which is made to be of an insulating natureat the top of the winding element in the slot, but is made to beconductive in the lower portions of the winding elements in the slots.This produces an equipotential surface so as to eliminate thetroublesome slot discharges which had occurred with previous insulationsystems. See also Canadian Patent 1,016,586--Aug. 30, 1977.

It is therefore concluded that the problem of stabilizing windingelements in the slots of dynamoelectric machines is an ongoing one whichhas been attacked from many directions with various degrees of success.The prior art solutions have at times required tedious and laborioustasks of inserting wavy springs into the crevices between the windingelements and the slots. Ideally if a perfect winding element withprecise dimensions could be manufactured, and inserted into a perfectlymanufactured and aligned magnetic core slot with a predetermined degreeof interference in the fit between the two, the ultimate desire of themachine manufacturer would be achieved.

The machine designer must ultimately deal with the problem of thedifferent rates of thermal expansion which occur for the windingelements and the magnetic core material which houses the windingelements. The relative motion which results due to differing thermalexpansion rates and electromagnetic forces leads to a condition commonlyreferred to as "fretting".

SUMMARY OF THE INVENTION

It is to overcome the imperfections as set out above that this inventionfinds its application. An elastomeric sheet is lapped over and along thelength of the winding element which is to be inserted in a stator slotof a large AC dynamoelectric machine. In one mode of this invention astretching device is attached to the elastomeric sheet and the windingelement so that the elastomeric sheet is stretched around the two sidesand the bottom of the winding element by the stretching device so as toeffectively temporarily reduce the wall thickness of the elastomericsheet.

With the stretching device still attached to the winding element, thecombined winding element which is covered on three sides by thestretched elastomeric sheet is inserted into the machine slot. Thestretching device is now uncoupled from the elastomeric sheet and thesheet attempts to return to its former thickness but now is constrainedto the space remaining between the winding element and the slot wall.The elastomer sheet is now trimmed and the offal is discarded.

If a second winding element is to be inserted above the previouslyinserted winding element, the same process is repeated. Upon trimmingthe offal, an appropriate slot wedge is inserted to lock the windingelement(s) in place in the slot.

The elastomeric material may be loaded with a heat conductive substanceto improve the heat transfer from the winding element to the steelstructure surrounding the slot, the important feature being that theelasticity of the elastomeric member must not be impaired. It is alsoimportant that the addition of the heat transfer medium to theelastomeric material does not shorten or interfere with the life ormechanical properties of the elastomeric material. For higher voltageapplications, the material added to the elastomer material may includeconductive particles to reduce the resistivity of the elastomericmaterial, and if the particles are such as to increase the thermalconductivity across the thickness of the elastomeric material, theoverall function of the elastomeric material is enhanced beyond a simplewinding element securing device. It is also important that thecompression set of the elastomeric material is not increased by theaddition of the particulate material added to the elastomeric material.

Another mode of this invention is to provide a winding element which mayhave the elastomeric material in contact with one, two, or three sidesof the winding element. The elastomeric material is and stretchedlongitudinally along the length of the slot which the winding element isto be inserted until the desired reduction of the thickness of theelastomeric material is achieved. The winding element is inserted intothe magnetic core slot where the stretched elastomeric strip is locatedand the tension is removed from the elastomeric material so as to allowthe elastomeric material to increase in thickness to fill the gapbetween the winding element and the slot wall and "set" the windingelement in the slot. It will be apparent to those skilled in the artthat the elastomeric material may be applied to only one side of thewinding element or to all three sides, or two opposing sides, and theelastomeric material may not be required to be trimmed in the slotportions, but only at the end of the stator core where the winding endsprotrude.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective view of a stator bar for insertion intoa stator slot of a large dynamoelectric machine;

FIG. 2 is a perspective view of a section of the stator of a largedynamoelectric machine;

FIG. 3 is a perspective view of the device of this invention;

FIG. 4 shows the device of this invention being applied to a stator bar.

FIG. 5 shows an alternative form of this invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings and to FIG. 1 in particular, where astator bar 10 is shown which is ready to be installed into the stator ofa large dynamoelectric machine. Stator bar 10 is composed of manyconductors which are suitably insulated from each other by means of asuitable insulating medium. The stator bar has a heavy groundwallinsulation on the exterior thereof to provide suitable electricalisolation between the conductors and the ground plane of the slot inwhich the stator bar 10 is to be installed. Bar 10 has a pair of barredends 12 and 14 for connection to other electrical conductors once bar 10is installed in a stator core.

Although bar 10 is composed of many individual conductors and insulatingtape, etc. the end product is a very rigid solid member having a hardouter surface 16.

FIG. 2 illustrates a stator core 50 comprising many steel punchingswhich are stacked together to form a generally cylindraceous body havinga cylindrical bore in the interior thereof. On the interior bore surfaceare a plurality of radial slots 52 extending longitudinally in an axialdirection the entire length of the stator 50. Slots 52 are of such awidth that stator bar would normally require a pressing device to pushthe stator bar into the empty stator slot in order to overcome theinsertion resistance caused by the interference fit of the stator barwith the stator slot.

FIG. 3 shows the device 60 of this invention. Basically, the stretchingdevice 60 comprises an elongated flat base member 62 which has a Vshaped groove 64 extending the length of member 62. Member 62 ispreferably an elongated flat member about which is made to about thesame length as the slot portion of the stator bars to be used withdevice 60. Hingedly attached to member 62 is another elongated flatmember 66. In this instance, the members are hingedly attached by meansby hinge 68. Member 66 is made to cooperate with member 62 in thatmember 66 is of about the same length as member 62 and the member 66 hasa V-shaped protruding ridge 68 which fits into V-shaped groove 64 whenthe member 62 and 66 are closed. Devices 70 are used to close and lockmembers 62 and 66 in a closed pressing position. In this instance, theclosing devices are threaded fasteners, however any clamping devicewhich will press the two co-operating members 64 and 66 together willfulfil the requirements of device 60.

At each end of member 62 are a pair of bosses 72 and 74 to which havethreaded bores therein having a pair threaded hand screws 76 and 78threadedly received therein. Each hand screw is provided with a handle,80 and 82 and a U-shaped end 84 and 86 having openings 88 and 90 of suchsize as to receive a stator bar therein such as the bar illustrated as10 in FIG. 1. The openings 88 and 90 are of such width so as to be ableto comfortably slide over the bar 10 with no excessive slop.

FIG. 4 shows the device 60 of FIG. 3 in a partial perspective having anelastomeric membrane 100 received between members 64 and 68 with member84 and 86 straddling the ends of bar 10, the membrane 100 is wrappedaround the stator bar 10 for the entire length of the stator slots suchas 52 of FIG. 2. When membrane has been wrapped around the stator bar 10so that both sides 102 and 104 are securely and evenly clamped in device60, the tightening devices 70 are advanced to securely clamp the sides102 and 104 of membrane 100 between members 62 and 66 of device 60. TheV groove 64 in member 62 co-operates with the projecting ridge 68 tosecure the membrane prior to the stretching operation.

The hand screws 76 and 78 are advanced to stretch the membrane byextending the U-shaped brackets 84 and 86 away from members 62 and 66.When the membrane 100 is stretched sufficiently, the membrane encasedbar 10 may be inserted into a slot such as 52 of FIG. 2. The bar encasedmembrane may be precisely located and the hand screws 76 and 78retracted to relax membrane 100. As the elastomeric membrane attempts toreturn to its original thickness the space between the bar 10 and slot52 will prevent the membrane from expanding to its original thickness.The device 60 may be opened and removed from the bar 10. The membranemay be trimmed of its offal, and the bar 10 is now firmly gripped inslot 52 by expansion of thickness of membrane 100.

FIG. 5 shows an alternative form of the invention when it is desired tohave the winding element secured in the stator slot by only one or twoindividual strips of elastomeric material. In the example shown, theapparatus is used to insert the winding element into the magnetic corewith only a single stretched elastomeric member present.

A magnetic core member 50 is shown having slots 52 of which one isshown. A stretching device 150 is fixed to the end of the magnetic core50 by any convenient means, in this instance, by magnetic attraction.Base 152 is placed against core 50 so that slot 50 lies in gap 154 ofmember 150. An elastomeric strip 156 is wound around shaft 158 of device150. Shaft 158 is journalled in bearings 160 and 162 of the stretchingdevice 150. Shaft 158 extends below bearing 162 and a hand pulley 164 ismounted on shaft 158 for twisting shaft 158 to wind elastomeric strip156 about shaft 158. In this manner, the strip 156 is stretched in slot52. A device similar to device 150 is fastened to the other end ofmagnetic core 50 at the opposite end of slot 52. Both devices may havethe hand wheels 164 twisted to stretch the elastomeric strip the lengthof the slot 52.

A stator winding element, in this instance a stator bar 16, may now beinserted in slot 52 of magnetic core 50. When the winding element islocated in the slot 52 in the desired location, the locking device 168is removed from its aperture 170 in wheel 164. The strip is then allowedto relax as shaft 158 unwinds the end of the strip 156 and the thicknessof strip 156 increases to fill the space in the magnetic slot betweenthe winding element 16 and the slot wall. The ends of the relaxed stripare trimmed of offal when the devices 150 are removed from core member50. The devices 50 are then mounted on core 50 at the next availableslot, and the next elastomeric strip is stretched along the slot.

Membranes 100 and 156 may be composed by any temperature or chemicallystabilized elastomer which is stretchable to reduce its thickness butwhich has a memory which attempts to have the membrane return to itsoriginal thickness when the stretching stress is removed. The membrane100 may be loaded with a variety of finely divided materials such ascarbon, metallic or other inorganic particles to improve its electricaland heat conductivity. Other insulating materials such as siliconnitride, aluminum oxide or boron nitride may be used to improve the heattransfer capability of the membrane while improving its heat transfercharacteristics and providing corona partial discharge resistance. Theonly requirement is that membrane 100 must retain a memory so that whenthe stretching stress is removed the membrane actively attempts toachieve its original thickness.

We claim:
 1. A method of retaining a winding element in the slot of thestator of a large dynamoelectric machine comprising:placing anelastomeric material over said winding element such that the elastomericmaterial extends along the slot portion of said winding element andenvelops three surfaces of the winding element along the length thereof,and applying a stretching force to said elastomeric material to stretchthe elastomeric material sufficiently to reduce the thickness of saidelastomeric material surrounding the three sides of said windingelement, and inserting the winding element and the stretched elastomericmaterial into the stator slot of said machine, and removing thestretching force from said elastomeric material and allowing theelastomeric material to relax in the slot space and expand in width toretain the winding element in said slot.
 2. A method as claimed in claim1 wherein the stretching force is applied transversely to the length ofsaid stator bar.
 3. A method as claimed in claim 2 wherein theelastomeric material contains finely divided particles of a heatconductive material.
 4. A method as claimed in claim 3 wherein saidparticles are also electrically conductive.
 5. A method of retaining awinding element in the slot of a magnetic core of a large dynamoelectricmachine comprising:placing at least one sheet of a suitable stretchableelastomeric membrane against at least one surface of said windingelement, and applying a stretching force to said membrane to reduce thethickness of said membrane, whilst keeping the stretched membrane incontact with said surface, and inserting said winding element and saidcontacting stretched membrane in said slot, and releasing saidstretching force on said membrane to permit said membrane to assume arelaxed state so as to expand and increase in thickness to retain saidwinding element in said slot.
 6. A method as claimed in claim 5 whereinthe force is applied axially along the length of said winding element.7. A method as claimed in claim 6 wherein a pair of elastomeric sheetsare placed against opposing surfaces of said winding element andstretched to reduce the thickness of said sheets, and placing thewinding element and said stretched sheets in said slot,and releasingsaid stretching force to permit said sheets to relax and increase inthickness to retain said winding element in said slot.
 8. A method asclaimed in claims 5, 6 or 7 wherein the membrane is loaded withparticles of a conductive material.